This invention relates to an electrochemical sensor capable of determining hydrogen peroxide concentrations in materials and also to an analyzer which uses such a sensor together with a different sensor.
Enzyme electrode sensors and electrochemical sensors for determining hydrogen peroxide concentrations in samples, provided with an immobilized membrane of oxidase that evolves hydrogen peroxide by an enzyme reaction, are equipped with membranes that permeate hydrogen peroxide. Sensors of these types, which detect hydrogen peroxide polarographically, are known, for example, from U.S. Pat. No. 4,240,889. A sensor typical of these types comprises a work electrode and a counter electrode, the former being usually formed of gold or platinum and the latter, of silver. It is common with such a sensor that the reaction EQU 4H.sup.+ +O.sub.2 +4e.fwdarw.2H.sub.2 O
takes place at its counter electrode.
This reaction exhibits a relatively stable potential when the electrode is made of silver, and merely applying a constant voltage to the silver counter electrode makes controlled potential electrolysis possible. For this reason counter electrodes of silver are preferred.
Attempts have been made to arrange an electrochemical sensor capable of detecting hydrogen peroxide together with another electrochemical sensor with a different purpose in a common flow passage. The latter may be, for example, an electrochemical sensor for urea-nitrogen determination equipped with a urease-immobilized-enzyme membrane. The urea-nitrogen-detecting sensor decomposes urea in the sample with ammonia and determines the resultant with the aid of an ammonium ion-selective electrode.
We have found that the urea-nitrogen-detecting sensor can seriously deteriorate in performance when it is located near another sensor in the same flow passage.